1. Field of the Invention
This invention relates to rotary-type tools used for material removing operations and particularly to wheels, discs, rings and the like employing a peripherally-disposed, fixed super-abrasive matrix as the material removal medium, such tools typically being employed for grinding, polishing or otherwise working glass, ceramics and the like. More specifically, the invention is directed to an improved design in such tools which effectively and inexpensively copes with a longstanding problem, namely the premature destruction, or undue foreshortening of the useful life of these costly tools caused by the use of faulty or incorrect procedures or implements during periodic dressing of the fixed super-abrasive matrix.
While the present invention will be described with particular reference to several embodiments of what are known in the trade as pencil edging wheels or U-wheels and which are typically used to grind radiused edges in the flat glass and related industries, it should be understood that the invention is not limited thereto. The concepts set forth herein can be readily adapted for use in connection with other material-removing super-abrasive rotary tools, including, but not limited to, seaming wheels and certain types of peripheral wheels, rings, discs and the like, as those skilled in the art will recognize in the light of the present disclosure.
As used herein, the term "super-abrasive" refers to abrasive media suitable for grinding, polishing or similarly working glass, ceramics and the like and having a hardness on the Knoop scale in excess of about 3,000 kg/mm.sup.2, preferably substantially in excess thereof. A comparison of Knoop and Mohs hardness values for conversion purposes, if necessary, is available in standard handbooks. Commercially available super-abrasives include natural and synthetic industrial diamonds and cubic boron nitride. For brevity of the description herein, reference will primarily be made to diamond, although, as already indicated, the invention is not limited thereto.
Super-abrasives contrast with "conventional abrasives" of more limited hardness, i.e., a hardness on the Knoop scale of less than about 3,000 kg/mm.sup.2. Commercially available conventional abrasives include, for example, garnet, silicon carbide, emery, aluminum oxide, zirconium oxide, cerium oxide, and the like.
The term "fixed", as used herein in connection with either super-abrasives or conventional abrasives, refers to the disposition of the abrasive particles in solid or bonded form, in contrast to being disposed in particulate or powdered form or dispersed in a liquid slurry or similar fluid medium. Thus, in the case of pencil edging wheels the super-abrasive particles are bonded by means of a binder to form an annularly-shaped grinding matrix with a predetermined configured outer periphery, as more fully described hereinafter.
2. Description of the Prior Art
For many years, pencil edging wheels or U-wheels have been used extensively to grind radiused surfaces in flat glass, a particularly extensive application being the grinding of radiused edges on the window glass employed in the modern day automobile. Such pencil edging wheels typically comprise a flat, annular steel body or hub, the periphery or rim of which is radially-inwardly slotted, usually about the center plane, to provide an annular pocket or recess which holds and acts as a support structure for the aforementioned annular fixed super-abrasive grinding matrix. For purposes of high speed edging of automotive window glass, for example, such wheels are typically mounted in Sun rotary edge grinders, a product of Glass Machine Specialties, Inc., Toledo, Ohio, and rotate at speeds in the range of about 2000 to 4000 revolutions per minute. Commercially-available, water-containing coolants are usually employed during the grinding step.
The width of the slot in the wheel periphery containing the super-abrasive matrix is usually slightly greater than the edge of the workpiece to be ground. Because of the high cost of the super-abrasive, however, the excess width is normally kept to a minimum, that is, only that amount required for clearance purposes. The depth of the slot is such that it contains sufficient super-abrasive matrix so as to assure long and economic wheel life despite normal peripheral wear and periodic dressings of the matrix which occurs during the grinding operations, as discussed hereinafter. The exposed peripheral surface of the superabrasive is, of course, configured so as to produce the desired radius on the glass edge being ground, e.g., a smoothly-curved surface.
During a typical grinding operation, however, the super-abrasives at the peripheral working surface of the matrix become flattened or dulled whereby cutting rates are reduced and productivity in terms of linear inches ground, or volume of material removed, per unit time falls. Unexposed super-abrasive in the matrix with fresh cutting edges may be exposed for more efficient grinding by employing a relatively-soft bonding material in the matrix, whereby the grinding operation itself results in progressive wearing away of the bond so as to gradually release the flattened or dulled diamond. This approach, however, is difficult to control, may result in shortened wheel life and is not considered economic or otherwise desirable.
Instead, a relatively wear-resistant or hard bond for the super-abrasive is preferably employed to assure long wheel life, and the matrix is periodically "dressed" so as to wear away some of the bond under controlled conditions and thereby remove the dulled diamond and expose the cutting edges of fresh super-abrasive to the workpiece. It is this "dressing" operation, however, which has given rise to the problem which has long plagued the industry and to which the present invention is directed.
In a typical dressing operation, a dressing wheel or stick is applied to the working surface as it spins so as to selectively wear away the bond. Such dressing wheels or sticks typically comprise fixed conventional abrasives capable of attacking the bond but having little effect upon the super-abrasive itself, e.g., aluminum oxide or silicon carbide. The selective removal of the bond adjacent the working surface releases dulled or flattened super-abrasive and exposes the sharp cutting surfaces of fresh super-abrasive to the workpiece.
The dressing wheel or stick must not, however, contact the adjacent rim of the wheel which supports and buttresses the super-abrasive matrix. As compared with the matrix, the rim structure, which is typically steel, is highly wear prone. Thus, if substantial or prolonged contact of the dressing medium and rim takes place, the rim will be rapidly and selectively eroded away, particularly at the interface with the super-abrasive matrix, leaving adjacent edge portions of the super-abrasive matrix exposed and unsupported. This unsupported matrix is prone to cracking, chipping and other undesired breakage under high speed grinding conditions. This not only damages or destroys the very costly grinding wheel but may also damage or destroy the workpiece.
Dressing machines are available which carefully align the dressing medium with the super-abrasive surface to be dressed without risk of contacting the supporting steel. The effort and down time associated with removing the grinding wheel from the grinding machine for dressing purposes, as well as the cost of such dressing machines, often renders such controlled dressing efforts impractical and uneconomical. This is particularly true when the grinding machine operator is paid at a piece rate and wishes to minimize down time and to maximize cutting rates. Thus, the operator typically uses a dressing stick and hand-dresses the rotating grinding wheel in the grinding machine itself. Unfortunately, because of operator carelessness or neglect or limited visual or manual accessibility to the grinding surface to be dressed, the dressing stick is often applied to the pencil edging wheel at an angle, off center, or otherwise canted in such a fashion that it contacts the supporting rim, as well as the super-abrasive matrix. As a result, portions of the supporting rim adjacent the matrix are rapidly worn away, often to the point where the grinding operation must be prematurely halted and the damaged wheel replaced. Similar damage is incurred, even if a dressing stick is correctly aligned, when an operator employs a dressing stick having a thickness greater than the width of the rim slot.
To cope with these problems, efforts have been made to educate operators as to the nature of the problem and to train them to use dressing sticks of correct thickness and to use care in properly aligning the dressing stick with the super-abrasive surface. Such efforts have only been partially successful, particularly in the case of operators of modern edge grinding machines with restricted accessibility to the working surface of the wheel because of limited space, the presence of safety guards and the like.
Another possible solution is to increase the width of the annular slot in the wheel rim and commensurately increase the amount of super-abrasive matrix therein so that even if a dressing stick is misaligned or if a dressing stick of excess thickness is used, contact with the supporting rim structure is avoided or seldom occurs. This solution, however, is not desirable from an economic standpoint because of the very high cost of the extra super-abrasive matrix required to fill such extra wide rim slot.
As a result, the industry has been plagued for many years with the aforementioned problems, and wheel manufacturers are confronted with excessive returns. This leads to demands for credit when apparent wheel life falls far short of what the manufacturer considered good-faith representations.